The primary goal of this research is to use cultured rodent cells as a model to elucidate basic genetic processes of mammalian phyrimidine metabolism. A variety of mutants affecting both the biosynthetic and salvage pathways of pyrimidine nucleotides in Chinese hamster cells have been isolated. The work in the current year involved (1) the establishment of a thin layer chromatographic procedure which can separate the metabolites of the last two enzymatic steps in UMP biosynthesis and (2) the development of various selection strategies for the isolation of mutants in cytidine triphosphate synthesis. Characterization of the molecular defects and genetic analysis such as reversion, genetic complementation and gene mapping of these mutants are in progress. Finally, possible regulatory mutants of pyrimidine metabolism will be sought and their mode of action and genetic behavior will be investigated.